This invention relates to a construction of a transparent sheet material suitable for making transparencies in plain paper electrostatic copiers. More particularly, it relates to a transparent sheet which utilizes a layer of an electrically conductive prime coat to minimize jamming of the sheet in an electrostatic copier.
As is well known, transfer electrostatic copying commonly involves imparting a uniform electrostatic charge, either positive or negative, depending on the specific machine under consideration, to a photoconducting surface that will hold a charge only in the dark, such as a selenium-coated drum. The charge may be imparted to the photoconducting surface by passing it under a series of corona-discharge wires in the dark. The photoconducting surface is then exposed through a lens system to a document or article bearing the image which is to be reproduced. In areas where light strikes the photoconducting surface, the charge is dissipated and flows off through a conducting support to ground, with the electrostatic charge remaining largely intact in the image areas. Next, oppositely charged toner material is brought into contact with the photoconducting surface, and the toner clings by electrostatic attraction to the charged areas of the surface. A sheet which is to receive the image is placed over the toner image, and is given a charge, such as by means of corona-discharge wires. As a result, a large portion of the charged toner on the photoconducting surface is transferred to the sheet. Finally, the toner is fused to the sheet by application of heat, pressure, or a combination of both.
When transparent, polymeric sheets are imaged in a conventional electrostatic copying machine, static charge on the surfaces of the sheets causes them to jam the machine or to pass through the machine without having an image formed thereon. Jamming can be caused by multiple feeding of sheets, i.e. more than one sheet entering the imaging zone of the copier at the same time. Multiple feeding can result from two or more sheets clinging together on account of static charge or excessively high coefficient of friction. While excessively high coefficient of friction can be reduced by proper selection and/or treatment of the surface material of the transparency sheet, it is desired to provide treatment to the transparent sheet material to reduce static charge, thus resulting in fewer jams and fewer unimaged sheets.
Sheets formed of polymeric material can acquire static charge in several ways. Static electricity is generated during the extrusion, coating, and sheeting steps employed in preparing the sheets. Surface ions, from surrounding air, can induce static charge on the surface of the sheet. Ions or electrons may also be present within the backing of coated sheets or within the coatings themselves. Finally, there may be a dipole charge resulting from differences in polarity of portions of the polymeric chain forming the polymeric sheet.